Sensors such a synchors, resolvers, linearly variable differential transformers (LVDT), rotary variable differential transformers (RVDT), linear Hall sensors, capacitive position sensors and others are utilized extensively across numerous industries wherever accurate sensing of position is needed. Many avionic applications require very accurate position information to control many of the aircraft surfaces such as flaps, ailerons, and rudders. This position information is fed back to a controller, which drives the surface to the command.
Most of the positioning sensors are magnetic and require AC excitation (sinusoidal waveform) to energize the device. The position information is contained in the amplitude of the sinusoidal waveform output from the sensor. One method of extracting the peak information from the amplitude varying sinusoidal waveform uses a phase locked loops (PLL). This technique relies on adjusting the output of a voltage-controller oscillator (VCO) until it locks in frequency and phase to the input signal. PLL are sensitive to phase delays, which cause circuit instability. In addition, PLLs are inherently noisy due to the VCO running asynchronously with other signals on the board.
Another technique to obtain the amplitude of a sinusoidal waveform from the above mentioned sensors is to either diode rectify and filter the signals or to use phase sensitive demodulators. These circuits use filters to obtain an average over many cycles of the input to arrive at the amplitude. Filters with long time constants are needed to obtain high accuracy. The time delay associated with these filters introduce significant unwanted delays in the overall system.